By counting back from the year a living tree was cut, it is possible to determine the date of tree rings with reasonable accuracy.

Events like fires add "marker layers" to tree rings.
Because most of the trees within a limited area will
show the scars from the same fires, it is possible to
compare dates across several trees. This process of
crossdating allows researchers to establish a timeline
or chronology.

Courtesy of Laboratory of Tree-Ring
Research, University of Arizona

This cross section of a tree reveals several dated fire scars. By establishing when fires occurred, researchers can date other wood samples that show a similar pattern of fire scars.

3.
Other than forest fires, what else might tree rings be
used to date?